Abstract
The quartz grains in many metamorphic rocks tend to be elongate parallel to the c-axis. Recently a similar elongation has been observed in the quartz grains of unmetamorphosed sandstones; also another elongation parallel to the unit rhombohedron. Current explanations ascribe these elongations to fractures parallel to these directions and differential abrasion during transport. To check these explanations three sets of experiments were carried out, with the following results: (1) There was a decided tendency for some samples of quartz to fracture parallel to the unit rhombohedron, but no sample showed a pronounced fracture parallel to the c-axis. (2) Quartz grains from weathered (but undisturbed) quartzose igneous and metamorphic rocks show a tendency to be elongate parallel to prism and unit rhombohedral faces. (3) Abrasion tests on oriented prisms show that quartz is harder on prism faces than normal thereto. It is concluded that the elongation of quartz sand grains is due to original shape rather than to fracture and differential abrasion during transport.
